1. Field of the Invention
The present invention relates to the treatment of magnesium alloys for the purpose of improving their corrosion resistance.
2. Description of the Related Art
Magnesium alloys are of great benefit because of their lightness. Applications relate especially to the transport (automobile and aeronautical) industries, medical equipment and mobile telephony. One of the weak points of these materials is their sensitivity to corrosion. Corrosion protection can be obtained by depositing a coating on or by treating the surface. The protective coatings may be produced, for example, by chemical conversion or by anodizing in solutions containing salts of metals and metalloids.
The conversion treatments are carried out by immersing the magnesium alloy workpiece to be treated in an oxidizing solution, without applying electrical current. The treatment solutions most commonly used in the industry are solutions of alkali metal chromates or dichromates, which very substantially improve the corrosion resistance of the alloys. However, these treatments have a major drawback insofar as they involve solutions of not insignificant toxicity, because of the presence of CrVI.
It has therefore been endeavored to replace chromate or dichromate solutions with less toxic solutions. For example, a cerium nitrate solution has been used to treat pure magnesium workpieces (H. Ardelean, et al. Proceedings of the 38th Annual Conference of Metallurgists, Symposium on “Environmental Degradation of Materials and Corrosion Control in Metals”, Canada, (1999), Met. Soc., published by M. Elboudjdaini and E. Ghali, 1999, pp. 85–103). Using this treatment, a film containing cerium oxide forms on the surface of the magnesium, with the result that the cathode reaction is inhibited and anodic dissolution is reduced.
It is also known that treatment by a cerium salt solution improves the corrosion resistance of stainless steels (Y. C. Lu, et al., Corrosion Science, Vol. 37, No. 1, pp. 145–155, 1995) and of aluminum and its alloys (A. J. Davenport, et al., Corrosion Science, Vol. 32, No. 5/6, pp. 653–663, 1991). It is also known that the immersion of a CuMgAl2 intermetallic compound in a Ce(NO3)3 solution causes the formation of an Al/Ce oxide precipitate (J. D. Gorman, et al., Corrosion Science, Vol. 38, No. 11, pp. 1977–1990, 1996). It is also known to treat an aluminum alloy containing magnesium with an aqueous cerium chloride solution that furthermore includes hydrogen peroxide (A. E. Hughes, et al., Vol. 23, 540–550 (1995). However, the protective coatings obtained have a few drawbacks insofar as they are porous and that their adhesion is insufficient.